Lygus Bugs on Potatoes in the Pacific Northwest Josephine Antwi1, Silvia I

OREGON STATE UNIVERSITY EXTENSION SERVICE Lygus bugs on potatoes in the Pacific Northwest Josephine Antwi1, Silvia I. Rondon2, and Rodney Cooper3 ygus bugs feed on a wide range of cultivated crops. In the U.S. Pacific LNorthwest (PNW), lygus bugs are pests of economically important crops including small fruits (e.g., strawberry), tree crops (e.g., apple, peach, nectar- ine, and pear), legume and hay for seed (e.g., alfalfa, clover seed, and canola), and vegetables (e.g., carrots and radish). Although rarely a pest on potatoes in the past, lygus bugs’ current abundance and distribution are causing alarm to potato growers in the PNW. Numerous reports have been received from growers in Oregon and Washington, particularly those in the lower Columbia Basin. In addition to causing direct feeding Figure 1. Lygus bug feeding on a potato leaf. Insert: Lygus bugs are characterized by damage, lygus bugs may carry or vector a conspicuous heart shape on their backs. Photos: J. Antwi, OSU Irrigated Agricultural Entomology Program. pathogens. Thus, it is important to © Oregon State University. understand the impact lygus bugs have on potatoes to fully understand the implications for potato production in the PNW. eggs in potato plants. Lygus bug eggs are difficult to see with the naked eye when they are laid in stems Identifying lygus bugs or midribs of leaves, because females insert the Lygus bugs are Hemipteran insects with eggs entirely into the plant tissues, exposing only characteristic piercing-sucking mouthparts. Adults each egg’s flat cap cover (arrow in Figure 3, page 2). are about ¼ inch (6 mm) long, half as wide, somewhat When eggs are laid in leaves, they are easier to spot hunchbacked, flat on the abdomen, and oval in (Figure 3). In both instances, you will need hand shape (Figure 1). Lygus bugs are recognized by a lenses to make proper identification. Immature lygus conspicuous heart shape on the upper center of the bugs, usually pale green, hatch out after about 7 days back known as the scutellum (see insert in Figure 1). and then undergo five stages of development before Immature lygus bugs look somewhat similar to reaching the adult stage. adults, except immatures are smaller, do not have wings, and do not reproduce. Newly hatched lygus 1Entomology Postdoctoral Scholar, Hermiston Agricultural resemble aphids, but they can be distinguished from Research and Extension Center, Crop and Soil Sciences, aphids by their lack of cornicles (Figure 2, page 2). Oregon State University; 2Extension Entomologist and Professor, Hermiston Agricultural Research and Extension Females are slightly larger than males and lay light Center, Oregon State University; 3USDA-ARS, Yakima cream-colored, ⅟25-inch (1 mm) long, oval-shaped Agricultural Research Laboratory, Wapato, Washington EM 9173 • August 2017 2nd instar 4th instar Egg Cornicle Lygus bug life cycle Aphid Adult 5th instar Figure 2. Life cycle of lygus bugs. Immatures resemble aphids, except immature lygus lack the cornicles found on the abdomen of aphids. Photos: J. Antwi, OSU Irrigated Agricultural Entomology Program, © Oregon State University. Composition of lygus bugs in potato (Knight) (western tarnished plant bug) and Lygus fields in the PNW elisus (Van Duzee) (pale legume bug). Both seem to be common lygus bug species in the western U.S. Since 2015, the Irrigated Agricultural Entomology Lygus lineolaris (Palisot de Beauvois) (tarnished plant Program (IAEP) at the Hermiston Agricultural bug) is commonly found in the eastern U.S., where it Research and Extension Center (HAREC) has is a common pest on a wide range of crops including been conducting an area-wide lygus bug survey in cotton, strawberry, and alfalfa. commercial potato fields in the lower Columbia Basin. Two main Lygus species have been identified Lygus bug damage on potatoes (based on morphological features): Lygus hesperus Both adult and immature lygus bugs feed on plants by inserting their piercing-sucking stylets into leaves and stems of host plants. Lygus bugs feed using “lacerate or macerate and flush feeding,” which allows them to ingest high volumes of food. They digest their food extraorally (outside of the mouth) by secreting enzymes through their saliva that produce a liquefied “plant soup” before ingestion. In potatoes, tissues at the site of lygus bug feeding do not grow normally. Feeding usually results in leaf flagging (leaves shrivel and turn brown or die and eventually fall off) (Figure 4a, page 3). Abnormal tissue growth through lygus bug feeding also results Figure 3. Lygus bug egg inserted into a potato leaf. A flat cap in leaf deformation (Figure 4b, page 3). Feeding injury covers the egg. often appears as brown lesions or dead tissue on Photo: J. Antwi, OSU Irrigated Agricultural Entomology Program, © Oregon State University. stems, leaf petioles, and midribs (Figure 4c, page 3), 2 and sometimes plant sap oozes from the point of commercial potato fields in Oregon’s Columbia feeding (Figure 4d). Lygus bugs prefer to feed on Basin. In 2015 and 2016, the highest frequency the upper foliage of potatoes, leaving much of their of BLTVA infection in lygus occurred mid-season damage on terminal leaves. (i.e., June–July) compared to infection in May or August. Thus, preliminary results prove that lygus Lygus bugs as potential vectors of bugs can carry the pathogen, but the efficiency potato pathogens of transmission, if it occurs, is unknown. Further The extent to which lygus bug feeding damage experiments are underway to test the potential for affects potato tuber yield is still unknown. But there lygus bugs to transmit the pathogen to potatoes. are concerns that lygus bugs could transmit potato Monitoring lygus bugs pathogens, specifically the beet leafhopper-transmitted virescence agent (BLTVA)—the causal agent of potato There are different techniques available for purple top disease (Figure 5, page 4). The pathogen sampling lygus bugs, including sticky cards, insect is thought to be transmitted to potatoes primarily by nets, and vacuums. Based on our experiments, the beet leafhopper (BLH), Circulifer tenellus Baker the inverted leaf blower (a.k.a. DVAC) is the most (Figure 5). However, lygus bugs have been observed effective sampling tool (Figure 6, page 4). The in association with potato plants expressing purple inverted leaf blower serves as a vacuum where the top symptoms, and some growers have a firm belief open end is covered with a nylon mesh that works that lygus vectors this pathogen. as a trapping bag for insects and prevents them from clogging the vacuum system. Currently, we Through molecular analysis, we have confirmed recommend using the vacuum for 5 minutes, 5 to the presence of BLTVA in lygus bugs collected from 10 feet from field borders. a b c d Figure 4. Damage on potato caused by lygus bug feeding: (a) flagging of leaves, (b) leaf deformation, (c) brown lesions at feeding site, (d) plant sap oozing from feeding sites Photos: J. Antwi, OSU Irrigated Agricultural Entomology Program, © Oregon State University. 3 At this time, there is no economic threshold for develops, to vine kill, when vines start yellowing and lygus bugs on potatoes in the PNW.1 Even so, it is very leaves fall. Our 2015 and 2016 area-wide lygus survey important to monitor lygus bugs in the field to avoid any yielded high numbers of lygus from June to August, with potential yield losses due to excessive feeding damage. population peaks in July. Lygus bugs are abundant on potatoes throughout the Thorough scouting in the early stages of potato growing season from as early as vegetative growth development is important. This will allow for early stage, when the visible portion of the plant emerges and detection and management of lygus bugs in order to avoid population outbreaks and severe damage, especially on young plants. Managing lygus bugs on potatoes and alfalfa Monitoring lygus populations on surrounding alfalfa crops has been suggested, because preliminary results indicate that as alfalfa is harvested, lygus bugs migrate to nearby potato fields. In alfalfa, lygus bug populations can build up to high numbers. As many as 100 lygus bug adults and 500 nymphs per 100 square feet have been reported in mature alfalfa stands. Thus, if feasible, proper management of alfalfa should be practiced to reduce lygus movement into potato fields. Potato growers are encouraged to time scouting and management of lygus bugs to coincide with alfalfa cuts. Border crops of alfalfa have proven successful in controlling lygus bug populations in cotton fields in the southeastern and southwestern U.S. Besides serving as a trap crop for lygus bugs, alfalfa fields also function as a refuge for beneficial insect predators (Figure 7, page 5) and parasitoids. If left uncut, alfalfa strips can support beneficial insect populations that a b Figure 5. (a) Potato symptomatic for purple top disease. (b) Beet Figure 6. An inverted leaf blower or DVAC for collecting insects. leafhopper (Circulifer tenellus) is the primary vector of the The open end is covered with a mesh secured with a rubber pathogen that causes purple top disease. band. It works as a vacuum by sucking up and trapping insects in Photo 5a: S.I. Rondon, © Oregon State University. Photo 5b: G. Oldfield the mesh. (USDA, Bugwood.org). Photo: S.I. Rondon, © Oregon State University. 1 In Ontario, Canada, where L. lineolaris is prevalent on potatoes, 25 adults per 25 sweeps over crop foliage seems to be the established threshold. This threshold may be different from the one needed in the PNW. 4 will move into potato fields to suppress lygus bugs and other pests on potatoes. Quinoa (Chenopodium quinoa Willd.) also supports large populations of Lygus (Figure 8). In potatoes, chemical products such as Vydate Cl-V or permethrin kept immatures and adults at low levels.

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